Memory device and fabrication thereof

ABSTRACT

A semiconductor memory device. A trench capacitor disposed at a lower portion of a trench in a substrate, in which the trench capacitor comprises a filling electrode layer and a collar dielectric layer surrounding the filling electrode layer. The top of the collar dielectric layer is lower than top surface level of the filling electrode layer. A vertical transistor is disposed at the upper portion of the trench, comprising a doped region disposed in a portion of the trench adjacent to the trench. A buried conductive layer interposed between the vertical transistor and the trench capacitor, wherein the cross section of the buried conductive layer is H shaped. The trench capacitor and the doping region of vertical transistor are electrically connected through the H shaped buried conductive layer.

BACKGROUND

The invention relates to a semiconductor device and fabrication thereof,and in particular to a memory device and fabrication thereof.

With the wide application of integrated circuits (ICs), several kinds ofsemiconductor devices with-higher efficiency and lower cost are producedbased on different objectives. The dynamic random access memory (DRAM)is an example of an important semiconductor device. Basically, a DRAM isan integrated circuit that stores data in binary form (e.g., “1” or “0”)in a large number of cells.

Most DRAMs have one transistor and one capacitor in one DRAM cell. Thememory capacity of the DRAM has reached 256 megabits. Therefore, underincreasing integration the size of the memory cell and the transistormust shrink to yield DRAM with higher memory capacity and higherprocessing speed. A 3-D capacitor structure can itself reduce theoccupied area in the semiconductor substrate, thus, the 3-D capacitor,such as a deep trench capacitor, is applied to the fabrication of theDRAM of 64 megabits and above. Traditional DRAM with a plane transistorcovers larger areas of the semiconductor substrate and cannot satisfythe demand for high integration. Therefore, a vertical transistor whichcan save space is a trend in memory cell fabrication.

In general, when forming a memory device with vertical transistors andtrench capacitors, formation of filling electrode (top electrode) isfabricated by deposition and etching. The etching back typically furthercomprises an over etching to completely remove the etched films. Duringthe over etching step, divots may be formed on the filling electrode,and tip is formed on the interface between the filling electrode and aburied conductive layer to generate point discharge, thus, performanceof the memory device is affected. In addition, the buried conductivelayer is typically formed by filling a polysilicon layer into the regionbetween the filling electrode and sidewalls of the trench. This method,however, is likely to generate seams. Since the buried conductive layeris a connection between the vertical transistor and the trenchcapacitor, seams in the buried conductive layer affect performance andreliability of the memory device.

SUMMARY

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred illustrativeembodiments of the present invention, which provide semiconductor memorydevice and fabrication thereof.

An embodiment of the invention provides a method for forming a memorydevice. A substrate comprising a trench is provided, wherein a trenchcapacitor is disposed in a lower portion of the trench. The trenchcapacitor comprises a filling electrode layer and a collar dielectriclayer surrounding the filling electrode layer. The top of the collardielectric layer is lower than the top surface level the fillingelectrode layer. A barrier layer is conformally formed on the collardielectric layer, the filling electrode layer and sidewalls of thetrench. A conductive layer is conformally deposited on the barrierlayer. A buried strap definition layer is formed on the conductive layerover the filling electrode layer. A portion of the conductive layer overthe buried strap definition layer is removed using the buried strapdefinition layer as an etching stop.

Another embodiment of the invention provides a semiconductor memorydevice. A trench capacitor is disposed at lower portion of a trench in asubstrate, in which the trench capacitor comprises a filling electrodelayer and a collar dielectric layer surrounding the filling electrodelayer. Top of the collar dielectric layer is lower than top surfacelevel of the filling electrode layer. A vertical transistor is disposedat upper portion of the trench, comprising a doped region disposed in aportion of the trench adjacent to the trench. A buried conductive layeris interposed between the vertical transistor and the trench capacitor,wherein the cross section of the buried conductive layer is H shaped.The trench capacitor and the doped region of the vertical transistor areelectrically connected through the H shaped buried conductive layer.

DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIGS. 1 a˜1 d illustrate a method known to the inventor of forming amemory device comprising a vertical transistor and a trench capacitor.

FIGS. 2 a˜2 f are intermediate cross sections of a memory device of anembodiment of the invention.

DETAILED DESCRIPTION

The following description discloses the best-contemplated mode ofcarrying out the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

In this specification, expressions such as “overlying the substrate”,“above the layer”, or “on the film” simply denote a relative positionalrelationship with respect to the surface of the base layer, regardlessof the existence of intermediate layers. Accordingly, these expressionsmay indicate not only the direct contact of layers, but also, anon-contact state of one or more laminated layers.

FIGS. 1 a˜1 d illustrate a method known to the inventor of forming amemory device comprising a vertical transistor and a trench capacitor.This is not prior art for the purpose of determining the patentabilityof the present invention. This merely shows a problem found by theinventors.

As shown in FIG. 1 a, a pad oxide layer 20 and a pad nitride layer 30are formed on a silicon substrate 100, and then patterned byconventional lithography and etching to form an opening. The substrate100 is etched using patterned pad oxide and pad nitride layers 20 and 30as a hard mask to form a plurality of trenches (only one trench isexpressed for simplicity). Next, a bottom electrode 110 is formed in aportion of the substrate 100 adjacent to the lower portion of the trench104 by diffusion. A capacitor dielectric layer 108 is formed onsidewalls of lower portion of the trench 104, and a collar dielectriclayer 112 is formed on sidewalls of upper portion of the trench 104. Apolysilicon conductive layer is then filled into the trench 104 to actas a top electrode 113 (filling electrode layer) of a trench capacitor.

As shown in FIG. 1 b, the filling electrode layer 113 and the collardielectric layer 112 are etched back to a predetermined depth lower thanthe substrate 100 surface level. Next, a doped silicon layer (notshown), such as arsenic silicate glass (ASG), is formed on sidewalls ofthe trench 104, overlying the collar dielectric layer 112. A dopingregion 150 is formed in the substrate by a thermal process to act as asource/drain of a vertical transistor. Thereafter, the doped siliconlayer is removed.

As shown in FIG. 1 c, the collar dielectric layer 112 surrounding thefilling electrode layer 113 is etched by selective etching to achieve asurface lower than the filling electrode layer 113 surface level,forming a gap between the filling electrode layer 113 and sidewalls ofthe trench 104.

As shown in FIG. 1 d, the conductive layer is etched back to form aburied conductive layer 116 between the conductive layer 117 andsidewalls of the trench 104. Next, a top insulating layer 118 is formedon the filling electrode layer 113. A gate dielectric layer 130 isformed on sidewalls of the trench 104, overlying the top insulatinglayer 118. A gate conductive layer 132 is filled into the trench 104,overlying the top insulating layer 118. Consequently, a trench capacitorcomprising the top electrode 106 (filling electrode layer), thecapacitor dielectric layer 108 and the bottom electrode 110 is complete.The gate conductive layer 132 and the gate dielectric layer 130 areelements of a vertical transistor. The buried conductive layer 116between the filling electrode layer 106 and sidewalls of the trench 104,and overlying the collar dielectric layer 112 serves to connect thetrench capacitor and the vertical transistor.

As shown in FIG. 1 d, due to small size of the gap between the fillingelectrode layer 106 and sidewalls of the trench 104, seam is likely tobe generated during filling of the gap. The buried conductive layer 116is for connecting the trench capacitor and the vertical transistor, andconnection fails between the trench capacitor and the verticaltransistor are likely to occur when the buried conductive layer 116comprises seams 122.

Formation of the filling electrode (top electrode) is accomplished bydeposition and etching back, in which etching back typically furthercomprises an over etching step to completely remove the remaining film.Due to the over etching step, divots is likely to be formed on thefilling electrode 106, and tip 120 is formed on the interface betweenfilling electrode layer 106 and a buried conductive layer 122 togenerate point discharge, affecting performance of the memory device.

FIGS. 2 a—2 f are intermediate cross sections of a memory device of anembodiment of the invention. Referring to FIG. 2 a, a pad oxide layer202 and a pad nitride layer 204 are formed on a substrate 200, such as asilicon substrate, and then patterned by conventional lithography andetching to form an opening. The substrate 200 is etched using patternedpad oxide and pad nitride layers 202 and 204 as a hard mask to form aplurality of trenches (only one trench 206 is expressed forsimplifying). Next, a bottom electrode 208 is formed in a portion of thesubstrate 200 adjacent to the lower portion of the trench 206 by thermaldiffusion. A capacitor dielectric layer 210, such as a stack layer ofoxide-nitride-oxide, is formed on sidewalls of lower portion of thetrench 206. A collar dielectric layer 212 is formed on sidewalls ofupper portion of the trench 206.

A conductive material, such as polysilicon, is then filled into thetrench 206 to form a filling electrode layer 214 (top electrode) in thetrench 206. Both the collar dielectric layer 212 and the fillingelectrode layer 214 are etched back to a predetermined depth lower thanthe substrate surface level. Consequently, a trench capacitor 216comprising the buried electrode layer 208, the capacitor dielectriclayer 210 and the filling electrode layer 214 is complete. The collardielectric layer, such as silicon oxide, can prevent leakage of chargesaccumulated from the trench capacitor 216 to the substrate or a verticaltransistor formed in subsequent process. Next, a doped silicon layer(not shown), such as arsenic silicate glass (ASG), is formed onsidewalls of the trench 206, overlying the collar dielectric layer 212.A doping region 232 is formed in the substrate 200 by a thermal processto act as a source/drain of a vertical transistor. Thereafter, the dopedsilicon layer is removed.

As shown in FIG. 2 b, the collar dielectric layer 212 surrounding thefilling electrode layer 214 is etched by selective etching to be lowerthan top surface level of the filling electrode layer 214. The selectiveetching can be wet etching, such as immersion in HF, or dry etching, forexample using CH4 or CHF3 as a main etchant. Next, as shown in FIG. 2 c,a barrier layer 218, such as silicon nitride, is deposited on thefilling electrode layer 214, the collar dielectric layer 212 andsidewalls of the trench 206. A conductive layer 220, preferably about100Å˜500 Å thick, is conformally deposited on the barrier layer 218,filling the gap between the filling electrode layer 214 and sidewalls ofthe trench 206.

A buried strap definition layer 222 is formed on the conductive layer214 by deposition and etching back. In a preferred embodiment of theinvention, the buried strap definition layer 222 fills a portion of thetrench 206 covered with the conductive layer 214 overlying the fillingelectrode layer 214. Preferably, the buried strap definition layer 222comprises silicon oxide, and is formed by the following steps. A buriedstrap definition layer is conformably deposited on the conductive layer220 and fills the trench 206. The deposition mentioned herein preferablyis a technique capable of forming the buried strap definition layer withthicker bottom and thinner sidewalls. For example, the depositionpreferably is high density plasma deposition, HDP.

Next, an isotropic etching, such as immersion in HF, is achieved toremove a portion of the buried strap definition layer on the sidewallsof trench 206. Due to the thick bottom and thinner sidewalls of theburied strap definition layer, a portion of the buried strap definitionlayer 222 adjacent to the bottom of the conductive layer remains.

As shown in FIG. 2 d, an isotropic etching, such as immersion in HF andHNO3 or dry etching using Cl containing gas as an ethant, is performedusing the buried strap definition layer 222 as an etching stop to removea portion of the conductive layer 224 overlying the buried strapdefinition layer 222. Preferably, the etched conductive layer 222 issubstantially coplanar with the buried strap definition layer 222,forming an H shaped buried conductive layer 224 in cross section view.As shown in FIG. 2 e, a top insulating layer 226, such as silicon oxide,is formed on the buried strap definition layer 222 and the conductivelayer 224 by deposition and etching back, to isolate the trenchcapacitor and a vertical transistor, formed in subsequent steps. Next, agate dielectric layer 228, such as silicon oxide formed by thermalprocess, is formed on the sidewalls of the trench 206 overlying the topinsulating layer 226. As shown in FIG. 2 f, a gate conductive layer 230,such as polysilicon, is formed in the trench 206, overlying the topinsulating layer 226, to act as a gate controlling the verticaltransistor 250.

As shown in FIG. 2 f, a cross section of a memory device comprising avertical transistor and a capacitor, a substrate 200 comprises a trench206, and a trench capacitor 216 is disposed in lower portion of thetrench 206. A vertical transistor 250 is disposed in an upper portion ofthe trench 206. An H shaped buried conductive layer 224 is interposedbetween the vertical transistor 216 and the capacitor 250.

The trench capacitor 216 comprises filling electrode layer 214 (topelectrode), a collar dielectric layer 212 surrounding the fillingelectrode layer 214, capacitor dielectric layer 210 on sidewalls of thelower portion of the trench 206, and a buried electrode layer 208disposed in a portion of the substrate 200 adjacent to the lower portionof the trench 206. The vertical transistor 250 comprises a gateconductive layer 230, a gate dielectric layer 228 on sidewalls of upperportion of the trench 206 and a source/drain region 232 disposed in thesubstrate 200. The H shaped buried conductive layer 224 is interposedbetween the vertical transistor 250 and the trench capacitor 216,filling a gap between the filling electrode layer 214 and sidewalls ofthe trench 206. A buried strap definition layer 222 is disposed on the Hshaped buried conductive layer 224 and surrounded by top cornersthereof. Additionally, the H shaped buried conductive layer 224comprises a cavity filled by the buried strap definition layer 222. Thevertical transistor 250 is isolated from the trench capacitor 216 by atop insulating layer 226, wherein both are electrically connectedthrough top corners of H shaped buried conductive layer 224.

The trench capacitor 216 of the memory device of a preferred embodimentof the invention is electrically connected to the doped region 232 ofthe vertical transistor 250 through the H shaped buried conductive layer224. Consequently, when seams are generated in the ring shaped region260 between the filling electrode layer 214 and the collar dielectriclayer 212, affection of connection between the vertical transistor andthe trench capacitor is reduced. In addition, due to there not being anydirect connection between the filling electrode layer 214 and thevertical transistor 250, point discharge caused by indentation of theover-etched filling electrode layer 214 is eliminated.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

1: A method for forming a memory device, comprising: providing asubstrate comprising a trench, wherein a trench capacitor is disposed ina lower portion of the trench, the trench capacitor comprises a fillingelectrode layer and a collar dielectric layer surrounding the fillingelectrode layer, top of the collar dielectric layer is lower than thetop surface level the filling electrode layer; conformally forming abarrier layer overlying the collar dielectric layer, the fillingelectrode layer and sidewalls of the trench; conformally depositing aconductive layer overlying the barrier layer; forming a buried strapdefinition layer on the conductive layer over the filling electrodelayer; and removing a portion of the conductive layer over the buriedstrap definition layer using the buried strap definition layer as anetching stop. 2: The method for forming a memory device as claimed inclaim 1, further comprising: forming a top insulating layer overlyingthe buried strap definition layer; forming a gate dielectric layer onsidewalls of the trench over the top insulating layer; and filling agate conductive layer into the trench, overlying the top insulatinglayer. 3: The method for forming a memory device as claimed in claim 1,wherein the remained conductive layer is substantially coplanar with theburied strap definition layer. 4: The method for forming a memory deviceas claimed in claim 1, wherein the step of forming the buried strapdefinition layer comprising: conformally depositing a dielectric layer,filling the trench, wherein the dielectric layer comprises a thickerbottom portion and a thinner sidewall portion; and isotropically etchingthe dielectric layer to remove a portion of the dielectric layer onsidewalls of the trench, remaining a bottom portion, wherein the bottomportion of the dielectric layer acts as the buried strap definitionlayer. 5: The method for forming a memory device as claimed in claim 4,wherein depositing the dielectric layer is accomplished by high densityplasma deposition. 6: The method for forming a memory device as claimedin claim 4, wherein isotropically etching the dielectric layer isaccomplished by immersion in HF. 7: The method for forming a memorydevice as claimed in claim 1, wherein removing a portion of theconductive layer over the buried strap definition layer is accomplishedby isotropic etching. 8: The method for forming a memory device asclaimed in claim 7, wherein the isotropic etching comprises a wetetching using mixer of HNO3 and HF as an etchant, and a dry etchingusing Cl containing gas as a main reactive gas. 9: The method forforming a memory device as claimed in claim 1, wherein the trenchcapacitor further comprises a capacitor dielectric layer disposed on asidewalls of lower portion of the trench, and a buried electrode layerdisposed in a portion of the substrate adjacent to lower portion of thetrench. 10: The method for forming a memory device as claimed in claim1, wherein cross section of the remained conductive layer is H shaped.11: The semiconductor memory device as claimed in claim 1, wherein theburied strap definition layer comprises silicon oxide. 12-20. (canceled)